Polynucleotides encoding a class II cytokine receptor

ABSTRACT

Novel receptor polypeptides, polynucleotides encoding the polypeptides, and related compositions and methods are disclosed. The polypeptides comprise an extracellular domain of a cell-surface receptor that is expressed in kidneys, pancreas, prostate, adrenal cortex and nervous tissue. The polypeptides may be used within methods for detecting ligands that promote the proliferation and/or differentiation of these organs.

The present application is a divisional of U.S. patent application Ser. No. 08/943,087, now U.S. Pat. No. 5,945,511, which is a continuation-in-part of U.S. patent application Ser. No. 08/803,305 filed Feb. 20, 1997, which is now abandoned.

BACKGROUND OF THE INVENTION

Cytokines are soluble proteins that influence the growth and differentiation of many cell types. Their receptors are composed of one or more integral membrane proteins that bind the cytokine with high affinity and transduce this binding event to the cell through the cytoplasmic portions of the certain receptor subunits. Cytokine receptors have been grouped into several classes on the basis of similarities in their extracellular ligand binding domains. For example, the receptor chains responsible for binding and/or transducing the effect of interferons (IFNs) are members of the type II cytokine receptor family (CRF2), based upon a characteristic 200 residue extracellular domain. The demonstrated in vivo activities of these interferons illustrate the enormous clinical potential of, and need for, other cytokines, cytokine agonists, and cytokine antagonists.

SUMMARY OF THE INVENTION

The present invention fills this need by providing novel cytokine receptors and related compositions and methods. In particular, the present invention provides for an extracellular ligand-binding region of a mammalian Zcytor7 receptor, alternatively also containing either a transmembrane domain or both an intracellular domain and a transmembrane domain.

Within one aspect, the present invention provides an isolated polynucleotide encoding a ligand-binding receptor polypeptide. The polypeptide comprises a sequence of amino acids selected from the group consisting of (a) residues 30 through 250 of SEQ ID NO:2; (b) allelic variants of (a); and (c) sequences that are at least 80% identical to (a) or (b). Within one embodiment, the polypeptide comprises residues 30 through 250 of SEQ ID NO:2. Within another embodiment, the polypeptide encoded by the isolated polynucleotide further comprises a transmembrane domain. The transmembrane domain may comprise residues 251 through 274 of SEQ ID NO:2, or an allelic variant thereof. Within another embodiment, the polypeptide encoded by the isolated polynucleotide further comprises an intracellular domain, such as an intracellular domain comprising residues 275 through 553 of SEQ ID NO:2, or an allelic variant thereof. Within further embodiments, the polynucleotide encodes a polypeptide that comprises residues 1 through 553, 1 through 274, 1 through 250, 30 through 274 or 30 through 553 of SEQ ID NO:2. Within an additional embodiment, the polypeptide further comprises an affinity tag. Within a further embodiment, the polynucleotide is DNA. Also claimed are the isolated polypeptides encoded by these polynucleotides.

Within a second aspect of the invention there is provided an expression vector comprising (a) a transcription promoter; (b) a DNA segment encoding a ligand-binding receptor polypeptide, wherein the ligand-binding receptor polypeptide comprises a sequence of amino acids selected from the group consisting of: (i) residues 30 through 250 of SEQ ID NO:2; (ii) allelic variants of (i); and (iii) sequences that are at least 80% identical to (i) or (ii); and (c) a transcription terminator, wherein the promoter, DNA segment, and terminator are operably linked. The ligand-binding receptor polypeptide may further comprise a secretory peptide, a transmembrane domain, a transmembrane domain and an intracellular domain, or a secretory peptide, a transmembrane domain and an intracellular domain.

Within a third aspect of the invention there is provided a cultured eukaryotic cell into which has been introduced an expression vector as disclosed above, wherein said cell expresses a receptor polypeptide encoded by the DNA segment. Within one embodiment, the cell further expresses a necessary receptor subunit which forms a functional receptor complex. Within another embodiment, the cell is dependent upon an exogenously supplied hematopoietic growth factor for proliferation.

Within a fourth aspect of the invention there is provided an isolated polypeptide comprising a sequence selected from the group consisting of (a) residues 30, a valine, through residue 250, a lysine, of SEQ ID NO:2; (b) allelic variants of (a); and (c) sequences that are at least 80% identical to (a) or (b), wherein said polypeptide is substantially free of transmembrane and intracellular domains ordinarily associated with hematopoietic receptors. Also claimed are polypeptides comprised of a sequence defined by residues 30, a valine, through residue 274, a tyrosine; and a polypeptide comprised of a sequence defined by residues 30, a valine, through residue 553 an asparagine. Also claimed are the polypeptides and polynucleotides defined by the sequences of SEQ ID NOs: 13-60.

Within a further aspect of the invention there is provided a chimeric polypeptide consisting essentially of a first portion and a second portion joined by a peptide bond. The first portion of the chimeric polypeptide consists essentially of a ligand binding domain of a receptor polypeptide selected from the group consisting of (a) a receptor polypeptide as shown in SEQ ID NO:2; (b) allelic variants of SEQ ID NO:2; and (c) receptor polypeptides that are at least 80% identical to (a) or (b). The second portion of the chimeric polypeptide consists essentially of an affinity tag. Within one embodiment the affinity tag is an immunoglobulin F_(c) polypeptide. The invention also provides expression vectors encoding the chimeric polypeptides and host cells transfected to produce the chimeric polypeptides.

The present invention also provides for an isolated polynucleotide encoding a polypeptide selected from a group defined SEQ ID NO:2 consisting of residues 1 through 250, residues 1 through 274, residues 1 through 553, residues 2 through 250, residues 2 through 274, residues 2 through 553, residues 251 through 274, residues 251 through 553 and residues 275 through 553. Also claimed are the isolated polypeptide expressed by these polynucleotides.

The invention also provides a method for detecting a ligand within a test sample, comprising contacting a test sample with a polypeptide as disclosed above, and detecting binding of the polypeptide to ligand in the sample. Within one embodiment the polypeptide further comprises transmembrane and intracellular domains. The polypeptide can be membrane bound within a cultured cell, wherein the detecting step comprises measuring a biological response in the cultured cell. Within another embodiment, the polypeptide is immobilized on a solid support.

Within an additional aspect of the invention there is provided an antibody that specifically binds to a polypeptide as disclosed above, as well as an anti-idiotypic antibody which binds to the antigen-binding region of an antibody to Zcytor7.

In still another aspect of the present invention, polynucleotide primers and probes are provided which can detect mutations in the Zcytor7 gene. The polynucleotide probe should at least be 20-25 bases in length, preferably at least 50 bases in length and most preferably about 80 to 100 bases in length. In addition to the detection of mutations, these probes can be used to discover the Zcytor7 gene in other mammalian species. The probes can either be positive strand or anti-sense strands, and they can be comprised of DNA or RNA.

These and other aspects of the invention will become evident upon reference to the following detailed description and the attached drawing.

DETAILED DESCRIPTION OF THE INVENTION

The term “allelic variant” is used herein to denote any of two or more alternative forms of a gene occupying the same chromosomal locus. Allelic variation arises naturally through mutation, and may result in phenotypic polymorphism within populations. Gene mutations can be silent (no change in the encoded polypeptide) or may encode polypeptides having altered amino acid sequence. The term allelic variant is also used herein to denote a protein encoded by an allelic variant of a gene.

The term “expression vector” is used to denote a DNA molecule, linear or circular, that comprises a segment encoding a polypeptide of interest operably linked to additional segments that provide for its transcription. Such additional segments include promoter and terminator sequences, and may also include one or more origins of replication, one or more selectable markers, an enhancer, a polyadenylation signal, etc. Expression vectors are generally derived from plasmid or viral DNA, or may contain elements of both.

The term “isolated”, when applied to a polynucleotide, denotes that the polynucleotide has been removed from its natural genetic milieu and is thus free of other extraneous or unwanted coding sequences, and is in a form suitable for use within genetically engineered protein production systems.

“Operably linked”, when referring to DNA segments, indicates that the segments are arranged so that they function in concert for their intended purposes, e.g. transcription initiates in the promoter and proceeds through the coding segment to the terminator.

A “polynucleotide” is a single- or double-stranded polymer of deoxyribonucleotide or ribonucleotide bases read from the 5′ to the 3′ end. Polynucleotides include RNA and DNA, and may be isolated from natural sources, synthesized in vitro, or prepared from a combination of natural and synthetic molecules.

The term “promoter” is used herein for its art-recognized meaning to denote a portion of a gene containing DNA sequences that provide for the binding of RNA polymerase and initiation of transcription. Promoter sequences are commonly, but not always, found in the 5′ non-coding regions of genes.

The term “receptor” is used herein to denote a cell-associated protein, or a polypeptide subunit of such a protein, that binds to a bioactive molecule (the “ligand”) and mediates the effect of the ligand on the cell. Binding of ligand to receptor results in a conformational change in the receptor (and, in some cases, receptor multimerization, i.e., association of identical or different receptor subunits) that causes interactions between the effector domain(s) and other molecule(s) in the cell. These interactions in turn lead to alterations in the metabolism of the cell. Metabolic events that are linked to receptor-ligand interactions include gene transcription, phosphorylation, dephosphorylation, cell proliferation, increases in cyclic AMP production, mobilization of cellular calcium, mobilization of membrane lipids, cell adhesion, hydrolysis of inositol lipids and hydrolysis of phospholipids. The term “receptor polypeptide” is used to denote complete receptor polypeptide chains and portions thereof, including isolated functional domains (e.g., ligand-binding domains).

A “secretory signal sequence” is a DNA sequence that encodes a polypeptide (a “secretory peptide”) that, as a component of a larger polypeptide, directs the larger polypeptide through a secretory pathway of a cell in which it is synthesized. The larger polypeptide is commonly cleaved to remove the secretory peptide during transit through the secretory pathway.

A “soluble receptor” is a receptor polypeptide that is not bound to a cell membrane. Soluble receptors are most commonly ligand-binding receptor polypeptides that lack transmembrane and cytoplasmic domains. Soluble receptors can comprise additional amino acid residues, such as affinity tags that provide for purification of the polypeptide or provide sites for attachment of the polypeptide to a substrate, or immunoglobulin constant region sequences. Many cell-surface receptors have naturally occurring, soluble counterparts that are produced by proteolysis or translated from alternatively spliced mRNAs. Receptor polypeptides are said to be substantially free of transmembrane and intracellular polypeptide segments when they lack sufficient portions of these segments to provide membrane anchoring or signal transduction, respectively.

The present invention is based in part upon the discovery of a novel DNA sequence that encodes a protein having the structure of a cytokine receptor, including the conserved WSXWS motif (SEQ ID NO: 10). Analysis of the tissue distribution of the mRNA corresponding to this novel DNA showed that mRNA level was highest in pancreas, prostate, kidney and adrenal cortex followed by lower levels in testis, stomach, adrenal medulla and thymus. The receptor has been designated “Zcytor7”.

Cytokine receptors subunits are characterized by a multi-domain structure comprising a ligand-binding domain and an effector domain that is typically involved in signal transduction. Multimeric cytokine receptors include homodimers (e.g., PDGF receptor αα and ββ isoforms, erythropoietin receptor, MPL [thrombopoietin receptor], and G-CSF receptor), heterodimers whose subunits each have ligand-binding and effector domains (e.g., PDGF receptor αβ isoform), and multimers having component subunits with disparate functions (e.g., IL-2, IL-3, IL-4, IL-5, IL-6, IL-7, and GM-CSF receptors). Some receptor subunits are common to a plurality of receptors. For example, the AIC2B subunit, which cannot bind ligand on its own but includes an intracellular signal transduction domain, is a component of IL-3 and GM-CSF receptors. Many cytokine receptors can be placed into one of four related families on the basis of their structures and functions. Class I hematopoietic receptors, for example, are characterized by the presence of a domain containing conserved cysteine residues and the WSXWS motif (SEQ ID NO: 10). Additional domains, including protein kinase domains; fibronectin type III domains; and immunoglobulin domains, which are characterized by disulfide-bonded loops, are present in certain hematopoietic receptors. Cytokine receptor structure has been reviewed by Urdal, Ann. Reports Med. Chem. 26:221-228, 1991 and Cosman, Cytokine 5:95-106, 1993. It is generally believed that under selective pressure for organisms to acquire new biological functions, new receptor family members arose from duplication of existing receptor genes leading to the existence of multi-gene families. Family members thus contain vestiges of the ancestral gene, and these characteristic features can be exploited in the isolation and identification of additional family members.

Cell-surface cytokine receptors are further characterized by the presence of additional domains. These receptors are anchored in the cell membrane by a transmembrane domain characterized by a sequence of hydrophobic amino acid residues (typically about 21-25 residues), which is commonly flanked by positively charged residues (Lys or Arg). On the opposite end of the protein from the extracellular domain and separated from it by the transmembrane domain is an intracellular domain.

The novel receptor of the present invention, Zcytor7, is a class II cytokine receptor. These receptors usually bind to four-helix-bundle cytokines. Interleukin-10 and the interferons have receptors in this class (e.g., interferon-gamma alpha and beta chains and the interferon-alpha/beta receptor alpha and beta chains). Class II cytokine receptors are characterized by the presence of one or more cytokine receptor modules (CRM) in their extracellular domains. The CRMs of class II cytokine receptors are somewhat different than the better known CRMs of class I cytokine receptors. While the class II CRMs contain two type-III fibronectin-like domains, they differ in organization. In particular, they contain two WSXWS (SEQ ID NO: 10) motifs, one in each fibronectin III-like domain. These WSXWS (SEQ ID NO: 10) motifs, however, are less conserved than those found in class I CRMs.

Zcytor7, like all known class II receptors except interferon-alpha/beta receptor alpha chain, has only a single class II CRM in its extracellular domain. Zcytor7 appears to be a receptor for a helical cytokine of the interferon/IL-10 class. Using the Zcytor7 receptor we can identify ligands and additional compounds which would be of significant therapeutic value. Furthermore, the extracellular portion of Zcytor7 extending from residue 30, a valine, through residue 250 of SEQ ID NO: 2 can be expressed and used as a soluble receptor to down-regulate the effects of the ligand of Zcytor7.

As was stated above, Zcytor7 was initially identified by the overall homology to CRF2-4, an orphan Class II cytokine receptor. See Lutfalla G. et al. Genomics, 16: 366-373 (1993). Analysis of a human cDNA clone encoding Zcytor7 (SEQ ID NO: 1) revealed an open reading frame encoding 553 amino acids (SEQ ID NO:2) comprising an extracellular ligand-binding domain of approximately 221 amino acid residues (residues 30-250 of SEQ ID NO:2), a transmembrane domain of approximately 24 amino acid residues (residues 251-274 of SEQ ID NO:2), and an intracellular domain of approximately 279 amino acid residues (residues 275-553 of SEQ ID NO:2). Those skilled in the art will recognize that these domain boundaries are approximate and are based on alignments with known proteins and predictions of protein folding. Deletion of residues from the ends of the domains is possible.

Within preferred embodiments of the invention the isolated polynucleotides will hybridize to similar sized regions of SEQ ID NO: 1 or a sequence complementary thereto, under stringent conditions. In general, stringent conditions are selected to be about 5° C. lower than the thermal melting point (T_(m)) for the specific sequence at a defined ionic strength and pH. The T_(m) is the temperature (under defined ionic strength and pH) at which 50% of the target sequence hybridizes to a perfectly matched probe. Typical stringent conditions are those in which the salt concentration is at least about 0.02 M at pH 7 and the temperature is at least about 60° C. As previously noted, the isolated polynucleotides of the present invention include DNA and RNA. Methods for isolating DNA and RNA are well known in the art. It is generally preferred to isolate RNA from pancreas or prostate tissues although cDNA can also be prepared using RNA from other tissues or isolated as genomic DNA. Total RNA can be prepared using guanidine HCl extraction followed by isolation by centrifugation in a CsCl gradient (Chirgwin et al., Biochemistry 18:52-94, 1979). Poly (A)⁺ RNA is prepared from total RNA using the method of Aviv and Leder (Proc. Natl. Acad. Sci. USA 69:1408-1412, 1972). Complementary DNA (cDNA) is prepared from poly(A)⁺ RNA using known methods. Polynucleotides encoding ZCytor7 polypeptides are then identified and isolated by, for example, hybridization or PCR.

Those skilled in the art will recognize that the sequences disclosed in SEQ ID NOS:1 and 2 represent single alleles of the human ZCytor7 receptor. Allelic variants of these sequences can be cloned by probing cDNA or genomic libraries from different individuals according to standard procedures. Other specific embodiments include the polynucleotides and polypeptides defined by SEQ ID NOs: 13-60.

The present invention further provides counterpart receptors and polynucleotides from other species (“species orthologs”). Of particular interest are ZCytor7 receptors from other mammalian species, including murine, porcine, ovine, bovine, canine, feline, equine, and other primate receptors. Species orthologs of the human ZCytor7 receptor can be cloned using information and compositions provided by the present invention in combination with conventional cloning techniques. For example, a cDNA can be cloned using mRNA obtained from a tissue or cell type that expresses the receptor. Suitable sources of mRNA can be identified by probing Northern blots with probes designed from the sequences disclosed herein. A library is then prepared from mRNA of a positive tissue or cell line. A receptor-encoding cDNA can then be isolated by a variety of methods, such as by probing with a complete or partial human cDNA or with one or more sets of degenerate probes based on the disclosed sequences. A cDNA can also be cloned using the polymerase chain reaction, or PCR (Mullis, U.S. Pat. No. 4,683,202), using primers designed from the sequences disclosed herein. Within an additional method, the cDNA library can be used to transform or transfect host cells, and expression of the cDNA of interest can be detected with an antibody to the receptor. Similar techniques can also be applied to the isolation of genomic clones.

The present invention also provides isolated receptor polypeptides that are substantially homologous to the receptor polypeptide of SEQ ID NO: 2. By “isolated” is meant a protein or polypeptide that is found in a condition other than its native environment, such as apart from blood and animal tissue. In a preferred form, the isolated polypeptide is substantially free of other polypeptides, particularly other polypeptides of animal origin. It is preferred to provide the polypeptides in a highly purified form, i.e. greater than 95% pure, more preferably greater than 99% pure. The term “substantially homologous” is used herein to denote polypeptides having 50%, preferably 60%, more preferably at least 80%, sequence identity to the sequences shown in SEQ ID NO:2. Such polypeptides will more preferably be at least 90% identical, and most preferably 95% or more identical to SEQ ID NO:2. Percent sequence identity is determined by conventional methods. See, for example, Altschul et al., Bull. Math. Bio. 48: 603-616, 1986 and Henikoff and Henikoff, Proc. Natl. Acad. Sci. USA 89:10915-10919, 1992. Briefly, two amino acid sequences are aligned to optimize the alignment scores using a gap opening penalty of 10, a gap extension penalty of 1, and the “blossom 62” scoring matrix of Henikoff and Henikoff (ibid.) as shown in Table 1 (amino acids are indicated by the standard one-letter codes). The percent identity is then calculated as: $\frac{{Total}\quad{number}\quad{of}\quad{identical}\quad{matches}}{\begin{matrix} \left\lbrack {{length}\quad{of}\quad{the}\quad{longer}\quad{sequence}\quad{plus}\quad{the}\quad{number}\quad{of}\quad{gaps}} \right. \\ {{introduced}\quad{into}\quad{the}\quad{longer}\quad{sequence}\quad{in}\quad{order}\quad{to}\quad{align}\quad{the}} \\ \left. {{two}\quad{sequences}} \right\rbrack \end{matrix}} \times 100$ TABLE 1 A R N D C Q E G H I L K M F P S T W Y V A 4 R −1 5 N −2 0 6 D −2 −2 1 6 C 0 −3 −3 −3 9 Q −1 1 0 0 −3 5 E −1 0 0 2 −4 2 5 G 0 −2 0 −1 −3 −2 −2 6 H −2 0 1 −1 −3 0 0 −2 8 I −1 −3 −3 −3 −1 −3 −3 −4 −3 4 L −1 −2 −3 −4 −1 −2 −3 −4 −3 2 4 K −1 2 0 −1 −3 1 1 −2 −1 −3 −2 5 M −1 −1 −2 −3 −1 0 −2 −3 −2 1 2 −1 5 F −2 −3 −3 −3 −2 −3 −3 −3 −1 0 0 −3 0 6 P −1 −2 −2 −1 −3 −1 −1 −2 −2 −3 −3 −1 −2 −4 7 S 1 −1 1 0 −1 0 0 0 −1 −2 −2 0 −1 −2 −1 4 T 0 −1 0 −1 −1 −1 −1 −2 −2 −1 −1 −1 −1 −2 −1 1 5 W − −3 −4 −4 −2 −2 −3 −2 −2 −3 −2 −3 − 1 −4 −3 −2 11 Y −2 −2 −2 −3 −2 −1 −2 −3 2 −1 −1 −2 −1 3 −3 −2 −2 2 7 V 0 −3 −3 −3 −1 −2 −2 −3 −3 3 1 −2 1 −1 −2 −2 0 −3 −1 4

Sequence identity of polynucleotide molecules is determined by similar methods using a ratio as disclosed above.

Substantially homologous proteins and polypeptides are characterized as having one or more amino acid substitutions, deletions or additions. These changes are preferably of a minor nature, that is conservative amino acid substitutions (see Table 2) and other substitutions that do not significantly affect the folding or activity of the protein or polypeptide. Also claimedmall deletions of SEQ ID NO:2, typically of one to about 30 amino acids; and small amino- or carboxyl-terminal extensions, such as an amino-terminal methionine residue, a small linker peptide of up to about 20-25 residues, or a small extension that facilitates purification (an affinity tag), such as a poly-histidine tract, protein A (Nilsson et al., EMBO J. 4:1075, 1985; Nilsson et al., Methods Enzymol. 198:3, 1991), glutathione S transferase (Smith and Johnson, Gene 67:31, 1988), or other antigenic epitope or binding domain. See, in general Ford et al., Protein Expression and Purification 2: 95-107, 1991. DNAs encoding affinity tags are available from commercial suppliers (e.g., Pharmacia Biotech, Piscataway, N.J.). TABLE 2 Conservative amino acid substitutions Basic: arginine lysine histidine Acidic: glutamic acid aspartic acid Polar: glutamine asparagine Hydrophobic: leucine isoleucine valine Aromatic: phenylalanine tryptophan tyrosine Small: glycine alanine serine threonine methionine

Essential amino acids in the receptor polypeptides of the present invention can be identified according to procedures known in the art, such as site-directed mutagenesis or alanine-scanning mutagenesis (Cunningham and Wells, Science 244, 1081-1085, 1989; Bass et al., Proc. Natl. Acad. Sci. USA 88:4498-4502, 1991). In the latter technique, single alanine mutations are introduced at every residue in the molecule, and the resultant mutant molecules are tested for biological activity (e.g., ligand binding and signal transduction) to identify amino acid residues that are critical to the activity of the molecule. Sites of ligand-receptor interaction can also be determined by analysis of crystal structure as determined by such techniques as nuclear magnetic resonance, crystallography or photoaffinity labeling. See, for example, de Vos et al., Science 255:306-312, 1992; Smith et al., J. Mol. Biol. 224:899-904, 1992; Wlodaver et al., FEBS Lett. 309:59-64, 1992. The identities of essential amino acids can also be inferred from analysis of homologies with related receptors.

Multiple amino acid substitutions can be made and tested using known methods of mutagenesis and screening, such as those disclosed by Reidhaar-Olson and Sauer (Science 241:53-57, 1988) or Bowie and Sauer (Proc. Natl. Acad. Sci. USA 86:2152-2156, 1989). Briefly, these authors disclose methods for simultaneously randomizing two or more positions in a polypeptide, selecting for functional polypeptide, and then sequencing the mutagenized polypeptides to determine the spectrum of allowable substitutions at each position. Other methods that can be used include phage display (e.g., Lowman et al., Biochem. 30:10832-10837, 1991; Ladner et al., U.S. Pat. No. 5,223,409; Huse, WIPO Publication WO 92/06204) and region-directed mutagenesis (Derbyshire et al., Gene 46:145, 1986; Ner et al., DNA 7:127, 1988)

Mutagenesis methods as disclosed above can be combined with high-throughput screening methods to detect activity of cloned, mutagenized receptors in host cells. Preferred assays in this regard include cell proliferation assays and biosensor-based ligand-binding assays, which are described below. Mutagenized DNA molecules that encode active receptors or portions thereof (e.g., ligand-binding fragments) can be recovered from the host cells and rapidly sequenced using modern equipment. These methods allow the rapid determination of the importance of individual amino acid residues in a polypeptide of interest, and can be applied to polypeptides of unknown structure.

Using the methods discussed above, one of ordinary skill in the art can prepare a variety of polypeptides that are substantially homologous to residues 30 to 250 of SEQ ID NO:2 or allelic variants thereof and retain the ligand-binding properties of the wild-type receptor. Such polypeptides may include additional amino acids from an extracellular ligand-binding domain of a Zcytor7 receptor as well as part or all of the transmembrane and intracellular domains. Such polypeptides may also include additional polypeptide segments as generally disclosed above.

Polynucleotides, generally a cDNA sequence, of the present invention encode the above-described polypeptides. A cDNA sequence which encodes a polypeptide of the present invention is comprised of a series of codons, each amino acid residue of the polypeptide being encoded by a codon and each codon being comprised of three nucleotides. The amino acid residues are encoded their respective codons as follows.

-   -   Alanine (Ala) is encoded by GCA, GCC, GCG or GCT;     -   Cysteine (Cys) is encoded by TGC or TGT;     -   Aspartic acid (Asp) is encoded by GAC or GAT;     -   Glutamic acid (Glu) is encoded by GAA or GAG;     -   Phenylalanine (Phe) is encoded by TTC or TTT;     -   Glycine (Gly) is encoded by GGA, GGC, GGG or GGT;     -   Histidine (His) is encoded by CAC or CAT;     -   Isoleucine (Ile) is encoded by ATA, ATC or ATT;     -   Lysine (Lys) is encoded by AAA, or AAG;     -   Leucine (Leu) is encoded by TTA, TTG, CTA, CTC, CTG or CTT;     -   Methionine (Met) is encoded by ATG;     -   Asparagine (Asn) is encoded by AAC or AAT;     -   Proline (Pro) is encoded by CCA, CCC, CCG or CCT;     -   Glutamine (Gin) is encoded by CAA or CAG;     -   Arginine (Arg) is encoded by AGA, AGG, CGA, CGC, CGG or CGT;     -   Serine (Ser) is encoded by AGC, AGT, TCA, TCC, TCG or TCT;     -   Threonine (Thr) is encoded by ACA, ACC, ACG or ACT;     -   Valine (Val) is encoded by GTA, GTC, GTG or GTT;     -   Tryptophan (Trp) is encoded by TGG; and     -   Tyrosine (Tyr) is encoded by TAC or TAT.

It is to be recognized that according to the present invention, when a cDNA is claimed as described above, it is understood that what is claimed are both the sense strand, the anti-sense strand, and the DNA as double-stranded having both the sense and anti-sense strand annealed together by their respective hydrogen bonds. Also claimed is the messenger RNA (mRNA) encodes the polypeptides of the present invention, and which mRNA is encoded by the above the above-described cDNA. A messenger RNA (mRNA) will encode a polypeptide using the same codons as those defined above, with the exception that each thymine nucleotide (T) is replaced by a uracil nucleotide (U).

The receptor polypeptides of the present invention, including full-length receptors, receptor fragments (e.g. ligand-binding fragments), and fusion polypeptides can be produced in genetically engineered host cells according to conventional techniques. Suitable host cells are those cell types that can be transformed or transfected with exogenous DNA and grown in culture, and include bacteria, fungal cells, and cultured higher eukaryotic cells. Eukaryotic cells, particularly cultured cells of multicellular organisms, are preferred. Techniques for manipulating cloned DNA molecules and introducing exogenous DNA into a variety of host cells are disclosed by Sambrook et al., Molecular Cloning: A Laboratory Manual, 2nd ed., Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y., 1989, and Ausubel et al., ibid.

The polynucleotides of the present invention can be synthesized using gene machines. Currently the method of choice is the phosphoramidite method. If chemically synthesized double stranded DNA is required for an application such as the synthesis of a gene or a gene fragment, then each complementary strand is made separately. The production of short genes (60 to 80 bp) is technically straightforward and can be accomplished by synthesizing the complementary strands and then annealing them. For the production of longer genes (>300 bp), however, special strategies must be invoked, because the coupling efficiency of each cycle during chemical DNA synthesis is seldom 100%. To overcome this problem, synthetic genes (double-stranded) are assembled in modular form from single-stranded fragments that are from 20 to 100 nucleotides in length.

One method for building a synthetic gene requires the initial production of a set of overlapping, complementary oligonucleotides, each of which is between 20 to 60 nucleotides long. The sequences of the strands are planned so that, after annealing, the two end segments of the gene are aligned to give blunt ends. Each internal section of the gene has complementary 3′ and 5′ terminal extensions that are designed to base pair precisely with an adjacent section. Thus, after the gene is assembled, the only remaining requirement to complete the process is sealing the nicks along the backbones of the two strands with T4 DNA ligase. In addition to the protein coding sequence, synthetic genes can be designed with terminal sequences that facilitate insertion into a restriction endonuclease sites of a cloning vector and other sequences should also be added that contain signals for the proper initiation and termination of transcription and translation.

An alternative way to prepare a full-size gene is to synthesize a specified set of overlapping oligonucleotides (40 to 100 nucleotides). After the 3′ and 5′ extensions (6 to 10 nucleotides) are annealed, large gaps still remain, but the base-paired regions are both long enough and stable enough to hold the structure together. The duplex is completed and the gaps filled by enzymatic DNA synthesis with E. coli DNA polymerase I. This enzyme uses the 3′-hydroxyl groups as replication initiation points and the single-stranded regions as templates. After the enzymatic synthesis is completed, the nicks are sealed with T4 DNA ligase. For larger genes (≧1,000 base pairs), the complete gene sequence is usually assembled from double-stranded fragments that are each put together by joining four to six overlapping oligonucleotides (20 to 60 bp each). If there is a sufficient amount of the double-stranded fragments after each synthesis and annealing step, they are simply joined to one another. Otherwise, each fragment is cloned into a vector to amplify the amount of DNA available. In both cases, the double-stranded constructs are sequentially linked to one another to form the entire gene sequence. Because it is absolutely essential that a chemically synthesized gene have the correct sequence of nucleotides, each double-stranded fragment and then the complete sequence is characterized by DNA sequence analysis. See Glick, Bernard R. and Jack J. Pasternak, Molecular Biotechnology, Principles & Applications of Recombinant DNA, (ASM Press, Washington, D.C. 1994), Itakura, K. et al. Synthesis and use of synthetic oligonucleotides. Annu. Rev. Biochem. 53: 323-356 (1984), and Climie, S. et al. Chemical synthesis of the thymidylate synthase gene. Proc. Natl. Acad. Sci. USA 87:633-637 (1990).

A DNA sequence encoding a ZCytor7 receptor polypeptide can then be operably linked to other genetic elements required for its expression, generally including a transcription promoter and terminator, within an expression vector. The vector will also commonly contain one or more selectable markers and one or more origins of replication, although those skilled in the art will recognize that within certain systems selectable markers may be provided on separate vectors, and replication of the exogenous DNA may be provided by integration into the host cell genome. Selection of promoters, terminators, selectable markers, vectors and other elements is a matter of routine design within the level of ordinary skill in the art. Many such elements are described in the literature and are available through commercial suppliers.

To direct a Zcytor7 receptor polypeptide into the secretory pathway of a host cell, a secretory signal sequence (also known as a leader sequence, prepro sequence or pre sequence) is provided in the expression vector. The secretory signal sequence may be that of the receptor, or may be derived from another secreted protein (e.g., t-PA) or synthesized de novo. The secretory signal sequence is joined to the ZCytor7 DNA sequence in the correct reading frame. Secretory signal sequences are commonly positioned 5′ to the DNA sequence encoding the polypeptide of interest, although certain signal sequences may be positioned elsewhere in the DNA sequence of interest (see, e.g., Welch et al., U.S. Pat. No. 5,037,743; Holland et al., U.S. Pat. No. 5,143,830).

Cultured mammalian cells are preferred hosts within the present invention. Methods for introducing exogenous DNA into mammalian host cells include calcium phosphate-mediated transfection (Wigler et al., Cell 14:725, 1978; Corsaro and Pearson, Somatic Cell Genetics 7:603, 1981: Graham and Van der Eb, Virology 52:456, 1973), electroporation (Neumann et al., EMBO J. 1:841-845, 1982), DEAE-dextran mediated transfection (Ausubel et al., eds., Current Protocols in Molecular Biology, John Wiley and Sons, Inc., NY, 1987), and liposome-mediated transfection (Hawley-Nelson et al., Focus 15:73, 1993; Ciccarone et al., Focus 15:80, 1993. The production of recombinant polypeptides in cultured mammalian cells is disclosed, for example, by Levinson et al., U.S. Pat. No. 4,713,339; Hagen et al., U.S. Pat. No. 4,784,950; Palmiter et al., U.S. Pat. No. 4,579,821; and Ringold, U.S. Pat. No. 4,656,134. Suitable cultured mammalian cells include the COS-1 (ATCC No. CRL 1650), COS-7 (ATCC No. CRL 1651), BHK (ATCC No. CRL 1632), BHK 570 (ATCC No. CRL 10314), 293 (ATCC No. CRL 1573; Graham et al., J. Gen. Virol. 36:59-72, 1977) and Chinese hamster ovary (e.g. CHO-Ki; ATCC No. CCL 61) cell lines. Additional suitable cell lines are known in the art and available from public depositories such as the American Type Culture Collection, Rockville, Md. In general, strong transcription promoters are preferred, such as promoters from SV-40 or cytomegalovirus. See, e.g., U.S. Pat. No. 4,956,288. Other suitable promoters include those from metallothionein genes (U.S. Pat. Nos. 4,579,821 and 4,601,978) and the adenovirus major late promoter.

Drug selection is generally used to select for cultured mammalian cells into which foreign DNA has been inserted. Such cells are commonly referred to as “transfectants”. Cells that have been cultured in the presence of the selective agent and are able to pass the gene of interest to their progeny are referred to as “stable transfectants.” A preferred selectable marker is a gene encoding resistance to the antibiotic neomycin. Selection is carried out in the presence of a neomycin-type drug, such as G-418 or the like. Selection systems may also be used to increase the expression level of the gene of interest, a process referred to as “amplification.” Amplification is carried out by culturing transfectants in the presence of a low level of the selective agent and then increasing the amount of selective agent to select for cells that produce high levels of the products of the introduced genes. A preferred amplifiable selectable marker is dihydrofolate reductase, which confers resistance to methotrexate. Other drug resistance genes (e.g. hygromycin resistance, multi-drug resistance, puromycin acetyltransferase) can also be used.

Other higher eukaryotic cells can also be used as hosts, including insect cells, plant cells and avian cells. Transformation of insect cells and production of foreign polypeptides therein is disclosed by Guarino et al., U.S. Pat. No. 5,162,222; Bang et al., U.S. Pat. No. 4,775,624; and WIPO publication WO 94/06463. The use of Agrobacterium rhizogenes as a vector for expressing genes in plant cells has been reviewed by Sinkar et al., J. Biosci. (Bangalore) 11:47-58, 1987.

Fungal cells, including yeast cells, and particularly cells of the genus Saccharomyces, can also be used within the present invention, such as for producing receptor fragments or polypeptide fusions. Methods for transforming yeast cells with exogenous DNA and producing recombinant polypeptides therefrom are disclosed by, for example, Kawasaki, U.S. Pat. No. 4,599,311; Kawasaki et al., U.S. Pat. No. 4,931,373; Brake, U.S. Pat. No. 4,870,008; Welch et al., U.S. Pat. No. 5,037,743; and Murray et al., U.S. Pat. No. 4,845,075. Transformed cells are selected by phenotype determined by the selectable marker, commonly drug resistance or the ability to grow in the absence of a particular nutrient (e.g., leucine). A preferred vector system for use in yeast is the POT1 vector system disclosed by Kawasaki et al. (U.S. Pat. No. 4,931,373), which allows transformed cells to be selected by growth in glucose-containing media. Suitable promoters and terminators for use in yeast include those from glycolytic enzyme genes (see, e.g., Kawasaki, U.S. Pat. No. 4,599,311; Kingsman et al., U.S. Pat. No. 4,615,974; and Bitter, U.S. Pat. No. 4,977,092) and alcohol dehydrogenase genes. See also U.S. Pat. Nos. 4,990,446; 5,063,154; 5,139,936 and 4,661,454. Transformation systems for other yeasts, including Hansenula polymorpha, Schizosaccharomyces pombe, Kluyveromyces lactis, Kluyveromycesfragilis, Ustilago maydis, Pichia pastoris, Pichia methanolica, Pichia guillermondii and Candida maltosa are known in the art. See, for example, Gleeson et al., J. Gen. Microbiol. 132:3459-3465, 1986 and Cregg, U.S. Pat. No. 4,882,279. Aspergillus cells may be utilized according to the methods of McKnight et al., U.S. Pat. No. 4,935,349. Methods for transforming Acremonium chrysogenum are disclosed by Sumino et al., U.S. Pat. No. 5,162,228. Methods for transforming Neurospora are disclosed by Lambowitz, U.S. Pat. No. 4,486,533.

Transformed or transfected host cells are cultured according to conventional procedures in a culture medium containing nutrients and other components required for the growth of the chosen host cells. A variety of suitable media, including defined media and complex media, are known in the art and generally include a carbon source, a nitrogen source, essential amino acids, vitamins and minerals. Media may also contain such components as growth factors or serum, as required. The growth medium will generally select for cells containing the exogenously added DNA by, for example, drug selection or deficiency in an essential nutrient which is complemented by the selectable marker carried on the expression vector or co-transfected into the host cell.

Within one aspect of the present invention, a novel receptor is produced by a cultured cell, and the cell is used to screen for ligands for the receptor, including the natural ligand, as well as agonists and antagonists of the natural ligand. To summarize this approach, a cDNA or gene encoding the receptor is combined with other genetic elements required for its expression (e.g., a transcription promoter), and the resulting expression vector is inserted into a host cell. Cells that express the DNA and produce functional receptor are selected and used within a variety of screening systems.

Mammalian cells suitable for use in expressing ZCytor7 receptors and transducing a receptor-mediated signal include cells that express other receptor subunits which may form a functional complex with Zcytor7. These subunits may include those of the interferon receptor family or of other class II or class I cytokine receptors. It is also preferred to use a cell from the same species as the receptor to be expressed. Within a preferred embodiment, the cell is dependent upon an exogenously supplied hematopoietic growth factor for its proliferation. Preferred cell lines of this type are the human TF-1 cell line (ATCC number CRL-2003) and the AML-193 cell line (ATCC number CRL-9589), which are GM-CSF-dependent human leukemic cell lines. In the alternative, suitable host cells can be engineered to produce the necessary receptor subunit or other cellular component needed for the desired cellular response. For example, the murine cell line BaF3 (Palacios and Steinmetz, Cell 41: 727-734, 1985; Mathey-Prevot et al., Mol. Cell. Biol. 6: 4133-4135, 1986) or a baby hamster kidney (BHK) cell line can be transfected to express the necessary □ subunit (also known as KH97) as well as a ZCytor7 receptor. The latter approach is advantageous because cell lines can be engineered to express receptor subunits from any species, thereby overcoming potential limitations arising from species specificity. In the alternative, species orthologs of the human receptor cDNA can be cloned and used within cell lines from the same species, such as a mouse cDNA in the BaF3 cell line. Cell lines that are dependent upon one hematopoietic growth factor, such as GM-CSF, can thus be engineered to become dependent upon a ZCytor7 ligand.

Cells expressing functional receptor are used within screening assays. A variety of suitable assays are known in the art. These assays are based on the detection of a biological response in a target cell. One such assay is a cell proliferation assay. Cells are cultured in the presence or absence of a test compound, and cell proliferation is detected by, for example, measuring incorporation of tritiated thymidine or by calorimetric assay based on the metabolic breakdown of 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MT) (Mosman, J. Immunol. Meth. 65: 55-63, 1983). An alternative assay format uses cells that are further engineered to express a reporter gene. The reporter gene is linked to a promoter element that is responsive to the receptor-linked pathway, and the assay detects activation of transcription of the reporter gene. A preferred promoter element in this regard is a serum response element, or SRE (see, e.g., Shaw et al., Cell 56:563-572, 1989). A preferred such reporter gene is a luciferase gene (de Wet et al., Mol. Cell. Biol. 7:725, 1987). Expression of the luciferase gene is detected by luminescence using methods known in the art (e.g., Baumgartner et al., J. Biol. Chem. 269:29094-29101, 1994; Schenborn and Goiffin, Promega Notes 41:11, 1993). Luciferase activity assay kits are commercially available from, for example, Promega Corp., Madison, Wis. Target cell lines of this type can be used to screen libraries of chemicals, cell-conditioned culture media, fungal broths, soil samples, water samples, and the like. For example, a bank of cell-conditioned media samples can be assayed on a target cell to identify cells that produce ligand. Positive cells are then used to produce a cDNA library in a mammalian expression vector, which is divided into pools, transfected into host cells, and expressed. Media samples from the transfected cells are then assayed, with subsequent division of pools, re-transfection, subculturing, and re-assay of positive cells to isolate a cloned cDNA encoding the ligand.

A natural ligand for the ZCytor7 receptor can also be identified by mutagenizing a cell line expressing the receptor and culturing it under conditions that select for autocrine growth. See WIPO publication WO 95/21930. Within a typical procedure, BaF3 cells expressing ZCytor7 and the necessary additional subunits are mutagenized, such as with 2-ethylmethanesulfonate (EMS). The cells are then allowed to recover in the presence of IL-3, then transferred to a culture medium lacking IL-3 and IL-4. Surviving cells are screened for the production of a ZCytor7 ligand, such as by adding soluble receptor to the culture medium or by assaying conditioned media on wild-type BaF3 cells and BaF3 cells expressing the receptor.

An additional screening approach provided by the present invention includes the use of hybrid receptor polypeptides. These hybrid polypeptides fall into two general classes. Within the first class, the intracellular domain of Z-Cytor7, comprising approximately residues 275 to 553 of SEQ ID NO:2, is joined to the ligand-binding domain of a second receptor. It is preferred that the second receptor be a hematopoietic cytokine receptor, such as mpl receptor (Souyri et al., Cell 63: 1137-1147, 1990). The hybrid receptor will further comprise a transmembrane domain, which may be derived from either receptor. A DNA construct encoding the hybrid receptor is then inserted into a host cell. Cells expressing the hybrid receptor are cultured in the presence of a ligand for the binding domain and assayed for a response. This system provides a means for analyzing signal transduction mediated by ZCytor7 while using readily available ligands. This system can also be used to determine if particular cell lines are capable of responding to signals transduced by ZCytor7. A second class of hybrid receptor polypeptides comprise the extracellular (ligand-binding) domain of ZCytor7 (approximately residues 30 to 250 of SEQ ID NO:2) with an intracellular domain of a second receptor, preferably a hematopoietic cytokine receptor, and a transmembrane domain. Hybrid receptors of this second class are expressed in cells known to be capable of responding to signals transduced by the second receptor. Together, these two classes of hybrid receptors enable the identification of a responsive cell type for the development of an assay for detecting a Zcytor7 ligand.

Cells found to express the ligand are then used to prepare a cDNA library from which the ligand-encoding cDNA can be isolated as disclosed above. The present invention thus provides, in addition to novel receptor polypeptides, methods for cloning polypeptide ligands for the receptors.

The tissue specificity of Zcytor7 expression suggests a role in the development of the kidney, pancreas, prostate or nervous tissues. In view of the tissue specificity observed for this receptor, agonists (including the natural ligand) and antagonists have enormous potential in both in vitro and in vivo applications. Compounds identified as receptor agonists are useful for stimulating proliferation and development of target cells in vitro and in vivo. For example, agonist compounds are useful as components of defined cell culture media, and may be used alone or in combination with other cytokines and hormones to replace serum that is commonly used in cell culture. Agonists may be useful in specifically promoting the growth and/or development of nervous, pancreatic or prostate-derived cells in culture. Antagonists are useful as research reagents for characterizing sites of ligand-receptor interaction. In vivo, receptor agonists or antagonists may find application in the treatment of renal, neural, pancreatic or prostate diseases.

ZCytor7 may also be used within diagnostic systems for the detection of circulating levels of ligand. Within a related embodiment, antibodies or other agents that specifically bind to ZCytor7 can be used to detect circulating receptor polypeptides. Elevated or depressed levels of ligand or receptor polypeptides may be indicative of pathological conditions, including cancer.

ZCytor7 receptor polypeptides can be prepared by expressing a truncated DNA encoding the extracellular domain, for example, a polypeptide which contains residues 30 through 250 of a human ZCytor7 receptor (SEQ ID NO:2) or the corresponding region of a non-human receptor. It is preferred that the extracellular domain polypeptides be prepared in a form substantially free of transmembrane and intracellular polypeptide segments. For example, the C-terminus of the receptor polypeptide may be at residue 250 of SEQ ID NO:2 or the corresponding region of an allelic variant or a non-human receptor. To direct the export of the receptor domain from the host cell, the receptor DNA is linked to a second DNA segment encoding a secretory peptide, such as a t-PA secretory peptide. To facilitate purification of the secreted receptor domain, a C-terminal extension, such as a poly-histidine tag, substance P, Flag™ peptide (Hopp et al., Biotechnology 6:1204-1210, 1988; available from Eastman Kodak Co., New Haven, Conn.) or another polypeptide or protein for which an antibody or other specific binding agent is available, can be fused to the receptor polypeptide.

In an alternative approach, a receptor extracellular domain can be expressed as a fusion with immunoglobulin heavy chain constant regions, typically an F_(C) fragment, which contains two constant region domains and a hinge region but lacks the variable region. Such fusions are typically secreted as multimeric molecules wherein the F_(C) portions are disulfide bonded to each other and two receptor polypeptides are arrayed in closed proximity to each other. Fusions of this type can be used to affinity purify the cognate ligand from solution, as an in vitro assay tool, to block signals in vitro by specifically titrating out ligand, and as antagonists in vivo by administering them parenterally to bind circulating ligand and clear it from the circulation. To purify ligand, a ZCytor7-Ig chimera is added to a sample containing the ligand (e.g., cell-conditioned culture media or tissue extracts) under conditions that facilitate receptor-ligand binding (typically near-physiological temperature, pH, and ionic strength). The chimera-ligand complex is then separated by the mixture using protein A, which is immobilized on a solid support (e.g., insoluble resin beads). The ligand is then eluted using conventional chemical techniques, such as with a salt or pH gradient. In the alternative, the chimera itself can be bound to a solid support, with binding and elution carried out as above. Chimeras with high binding affinity are administered parenterally (e.g., by intramuscular, subcutaneous or intravenous injection). Circulating molecules bind ligand and are cleared from circulation by normal physiological processes. For use in assays, the chimeras are bound to a support via the F_(C) region and used in an ELISA format.

A preferred assay system employing a ligand-binding receptor fragment uses a commercially available biosensor instrument (BIAcore™, Pharmacia Biosensor, Piscataway, N.J.), wherein the receptor fragment is immobilized onto the surface of a receptor chip. Use of this instrument is disclosed by Karlsson, J. Immunol. Methods 145:229-240, 1991 and Cunningham and Wells, J. Mol. Biol. 234:554-563, 1993. A receptor fragment is covalently attached, using amine or sulfhydryl chemistry, to dextran fibers that are attached to gold film within the flow cell. A test sample is passed through the cell. If ligand is present in the sample, it will bind to the immobilized receptor polypeptide, causing a change in the refractive index of the medium, which is detected as a change in surface plasmon resonance of the gold film. This system allows the determination of on- and off-rates, from which binding affinity can be calculated, and assessment of stoichiometry of binding.

Ligand-binding receptor polypeptides can also be used within other assay systems known in the art. Such systems include Scatchard analysis for determination of binding affinity (see, Scatchard, Ann. NY Acad. Sci. 51: 660-672, 1949) and calorimetric assays (Cunningham et al., Science 253:545-548, 1991; Cunningham et al., Science 254:821-825, 1991).

A receptor ligand-binding polypeptide can also be used for purification of ligand. The receptor polypeptide is immobilized on a solid support, such as beads of agarose, cross-linked agarose, glass, cellulosic resins, silica-based resins, polystyrene, cross-linked polyacrylamide, or like materials that are stable under the conditions of use. Methods for linking polypeptides to solid supports are known in the art, and include amine chemistry, cyanogen bromide activation, N-hydroxysuccinimide activation, epoxide activation, sulfhydryl activation, and hydrazide activation. The resulting media will generally be configured in the form of a column, and fluids containing ligand are passed through the column one or more times to allow ligand to bind to the receptor polypeptide. The ligand is then eluted using changes in salt concentration or pH to disrupt ligand-receptor binding.

ZCytor7 polypeptides can also be used to prepare antibodies that specifically bind to ZCytor7 polypeptides. As used herein, the term “antibodies” includes polyclonal antibodies, monoclonal antibodies, single-chain antibodies and antigen-binding fragments thereof such as F(ab′)₂ and Fab fragments, and the like, including genetically engineered antibodies. Antibodies are defined to be specifically binding if they bind to a ZCytor7 polypeptide with a K_(a) of greater than or equal to 10⁷/M. The affinity of a monoclonal antibody can be readily determined by one of ordinary skill in the art (see, for example, Scatchard, ibid.).

Methods for preparing polyclonal and monoclonal antibodies are well known in the art (see for example, Sambrook et al., Molecular Cloning: A Laboratory Manual, Second Edition, Cold Spring Harbor, N.Y., 1989; and Hurrell, J. G. R., Ed., Monoclonal Hybridoma Antibodies: Techniques and Applications, CRC Press, Inc., Boca Raton, Fla., 1982). As would be evident to one of ordinary skill in the art, polyclonal antibodies can be generated from a variety of warm-blooded animals such as horses, cows, goats, sheep, dogs, chickens, rabbits, mice, and rats. The immunogenicity of a ZCytor7 polypeptide may be increased through the use of an adjuvant such as Freund's complete or incomplete adjuvant. A variety of assays known to those skilled in the art can be utilized to detect antibodies which specifically bind to ZCytor7 polypeptides. Exemplary assays are described in detail in Antibodies: A Laboratory Manual, Harlow and Lane (Eds.), Cold Spring Harbor Laboratory Press, 1988. Representative examples of such assays include: concurrent immunoelectrophoresis, radio-immunoassays, radio-immunoprecipitations, enzyme-linked immunosorbent assays (ELISA), dot blot assays, inhibition or competition assays, and sandwich assays.

Antibodies to ZCytor7 may be used for tagging cells that express the receptor, for affinity purification, within diagnostic assays for determining circulating levels of soluble receptor polypeptides, and as antagonists to block ligand binding and signal transduction in vitro and in vivo.

Anti-idiotypic antibodies which bind to the antigenic binding site of antibodies to Zcytor7 are also considered part of the present invention. The antigenic binding region of the anti-idiotypic antibody thus will mimic the ligand binding region of Zcytor7. An anti-idiotypic antibody thus could be used to screen for possible ligands of the Zcytor7 receptor. Thus neutralizing antibodies to Zcytor7 can be used to produce anti-idiotypic antibodies by methods well known in the art as is described in, for example, Sambrook et al., Molecular Cloning: A Laboratory Manual, Second Edition, (Cold Spring Harbor, N.Y., 1989); and Hurrell, J. G. R., Ed., Monoclonal Hybridoma Antibodies: Techniques and Applications, (CRC Press, Inc., Boca Raton, Fla., 1982).

Zcytor-7 maps 795.76 cR from the top of the human chromosome 6 linkage group on the WICGR radiation hybrid map. Relative to the centromere, its nearest proximal marker was CHLC.GATA32B03 and its nearest distal maker was SGC32063. The use of surrounding markers also helped position zcytor-7 in the 6q22-q23 region on the CHLC chromosome 6 version v8c7 integrated marker map. The locus where Zcytor7 maps onto chromosome 6 is a common breakpoint area in ALL (acute lymphoblastic leukemia) and NHL (non-Hodgkin lymphoma) as well as in AML (acute myelogenous leukemia) and CML (chronic myeloid leukemia). It is interesting to note that the MYB (avian myeloblastosis viral oncogene homolog) gene, which encodes proteins critical for hematopoetic. cell proliferation and development, appears to be less than 800 kB from zcytor7. The 6q-deletion breakpoints occur slightly distal to the MYB gene and although the neoplasms show high levels of MYB mRNA, the gene itself appears to be intact.

Thus Zcytor7 could be used to generate a probe that could allow detection of an aberration of the Zctyor7 gene in the 6q chromosome which may indicate the presence of a cancerous cell such as leukemic cells which may still be present in after chemical or radiation therapy. If the Zcytor7 gene is deleted by the chromosomal abnormality, only one copy can be used to determine whether one or two copies of the gene are present per nucleus, thus indicating the percentage cancerous cells might be present relative to normal cells. For further discussions on developing polynucleotide probes and hybridization see Current Protocols in Molecular Biology Ausubel, F. et al. Eds. (John Wiley & Sons Inc. 1991).

Pharmaceutical Compositions

Pharmaceutical compositions can be formulated which contain the soluble receptor, antibody or anti-idiotypic antibodies of the present invention. Generally included in such protein therapeutic compositions are buffers; surface adsorption inhibitors such as surfactants and polyols; and isotonic amounts of a physiologically acceptable salt. The composition may be formulated as an aqueous solution or a lyophilized powder. The latter is reconstituted prior to use with a pharmaceutically acceptable diluent such as sterile water for injection.

Examples of buffers which can be used for the above-described pharmaceutical compositions include low ionic strength, physiologically acceptable buffers that are effective within the pH range of 5.0-7.0. Such buffers include phosphate, acetate, citrate, succinate and histidine buffers.

Examples of surface adsorption inhibitors which can be used in the above-described pharmaceutical compositions include non-ionic surfactants and polyols. Non-ionic surfactants include polyoxyethylene sorbitan fatty acid esters, such as polysorbate 20 (polyoxyethylene sorbitan monolaurate), and the like. Other non-ionic surfactants useful in this regard include polyethylene oxides; sorbitan esters; polyoxyethylene alkyl ethers; and glycerides of fatty acid, including glyceryl monooleate and glyceryl monostearate. Polyols which can be used include polyethylene glycol, e.g. PEG 3350, mannitol, xylitol, sorbitol, inositol, and glycerol. In general, the surface adsorption inhibitor will be included within the composition at a concentration from 0.001% to 5%.

Physiologically acceptable salts are generally included in a protein therapeutic composition generally in an amount isotonic to human blood. Preferred salts in this regard include chloride salts such as NaCl, KCl, CaCl₂ and MgCl₂.

Albumin may also be included in the above-described pharmaceutical compositions. Human serum albumin is preferred for inclusion in pharmaceutical compositions intended for human use. Albumin is useful as an excipient in lyophilized compositions and acts as a stabilizer when included at a concentration of 0.1-1.0%. Albumin may useful as a surface adsorption inhibitor.

One or more preservatives may also be included in the pharmaceutical compositions of the present invention. Common preservative include methylparaben, propylparaben, benzyl alcohol, m-cresol, ethylmercurithiosalycilate, phenol, thimerosol and the like. Methods of formulation of pharmaceutical compositions are well known in the art and are disclosed, for example, in Remington's Pharmaceutical Sciences, Gennaro, ed., (Mack Publishing Co., Easton, Pa., 1996) Dosages

Therapeutic doses of the protein compositions of the present invention will generally be in the range of 0.1 to 100 μg/kg of patient per day with the exact dose determined by the clinician.

The invention is further illustrated by the following non-limiting examples.

EXAMPLE 1 Cloning of Zcytor7

Expressed sequence tag (EST) 277139 (SEQ ID NO:3) was identified. The cDNA clone (ID No. 50416) was obtained from the IMAGE consortium Lawrence Livermore National Laboratory through Genome Systems, Inc. The cDNA was supplied as an agar stab containing E. coli transfected with a plasmid having the cDNA of interest. The E. coli was streaked on an agar plate. The plasmid was designated pSL7139. The cDNA insert in plasmid pSL7139 was sequenced. The insert was determined to be 1231 bp in length, but was not a full length sequence.

A human testis cDNA template was made using a MARATHON™ cDNA Amplification Kit (Clontech Laboratories, Inc., Palo Alto, Calif.) according to the supplier

s instructions. A 5

RACE reaction was used to obtain a full-length cDNA. The RACE reaction was carried out in two reactions employing two sets of primers. Reaction I (outer nest), using primers ZC11,107 (SEQ ID NO:4) and AP-1 (SEQ ID NO: 5) (Clontech Laboratories) was run for 35 cycles at 98° C. for 20 seconds, 45° C. for 20 seconds; 68° for 4 minutes and a final extension time of 10 minutes at 68° C. One μl of a 1:100 dilution of the reaction product was used as a template in reaction II (inner nest). Primers were ZC11,108 (SEQ ID NO:6) and AP-2 (SEQ ID NO:7) (Clontech Laboratories). The reaction was run at 98° C. for 30 seconds, and 30 cycles each cycle being comprised of 98° C. for 28 seconds; 43° C. for 20 seconds; and 68° C. for 3.5 minutes with a final extension at 68° C. for 10 minutes.

The product of the inner nest RACE reaction was subcloned using a PCR-SCRIPT™ kit (Stratagene Cloning Systems, La Jolla, Calif.) to prepare the plasmid pSLR7-1. Sequence analysis of this plasmid indicated that the 5

RACE-generated sequence extended the sequence of pSL7139 by 555 bp.

Full-length cDNA was obtained by screening a λZAP® human testis cDNA library using a probe that was generated by PCR primers ZC11,526 (SEQ ID NO:9) and ZC11,108 (SEQ ID NO:6) and pSLR7-1 as template and then re-amplified. The resulting probe was purified through recovery from low-melt agarose gel electrophoresis and was labeled with 32P-α-dCTP using a MEGAPRIME™ labeling kit (Amersham Corp., Arlington, Heights, Ill.). The labeled probe was purified on a push column (NUCTRAP® probe purification column; Stratagene Cloning Systems).

The first strand cDNA reaction contained 15 μl of human testis twice poly d(T)-selected poly (A)⁺ mRNA (Clontech Laboratories) at a concentration of 1.0 μg/μl, and 3 μl of 20 pmole/μl first strand primer ZC6091 (SEQ ID NO:8) containing an Xho I restriction site. The mixture was heated at 70° C. for 4 minutes and cooled by chilling on ice. First stand cDNA synthesis was initiated by the addition of 12 μl of first strand buffer (5×SUPERSCRIP™ buffer; Life Technologies, Gaithersburgh, Md.), 6 μl of 100 mM dithiothreitol, 3 μl of deoxynucleotide triphosphate solution containing 10 mM each of dTTP, dATP, dGTP, and 5-methyl-dCTP (Pharmacia LKB Biotechnology, Piscataway, N.J.) to the RNA-primer mixture. The reaction mixture was incubated at 37° C. for 2 minutes, followed by the addition of 15 μl of 200 U/μl Rnase H⁻ reverse transcriptase (SUPERSCRIPT II^(<<); Life Technologies). The efficiency of the first strand synthesis was analyzed in a parallel reaction by the addition of 5 μCi of ³²P-αdCTP to 5 μl aliquot from one of the reaction mixtures to label the reaction for analysis. The reactions were incubated at 37° C. for 10 minutes, 45° C. for 1 hour, then incubated at 50° C. for 10 minutes. Unincorporated ³²P-αdCTP in the labeled reaction and the unincorporated nucleotides and primers in the unlabeled first strand reactions were removed by chromatography on a 400 pore size gel filtration column (Clontech Laboratories). The length of labeled first strand cDNA was determined by agarose gel electrophoresis.

The second strand reaction contained 120 μl of the unlabeled first strand cDNA, 36 μl of 5×polymerase I buffer (125 mM Tris: HCl, pH 7.5, 500 mM KCl, 25 mM MgCl₂, 50 mM (NH₄)₂SO₄)), 2.4 μl of 100 mM dithiothreitol, 3.6 μl of a solution containing 10 mM of each deoxynucleotide triphosphate, 6 μl of 5 mM β-NAD, 3.6 μL of 3 U/μl E. coli DNA ligase (New England Biolabs), 9 μl of 10 U/μl E. coli DNA polymerase I (New England Biolabs), and 1.8 μl of 2 U/μl RNase H (life Technologies). A 10 μl aliquot from one of the second strand synthesis reactions was labeled by the addition of 10 μCi ³²P-αdCTP to monitor the efficiency of second strand synthesis. The reactions were incubated at 16° C. for two hours, followed by the addition of 15 μl T4 DNA polymerase (10 U/μl, Boerhinger Mannheim, Indianapolis, Ind.) and incubated for an additional 5 minutes at 16° C. Unincorporated ³²P-αdCTP in the labeled reaction was removed by chromatography through a 400 pore size gel filtration (Clontech Laboratories) before analysis by agarose gel electrophoresis. The unlabeled second strand reaction was terminated by the addition of 20 μl 0.5 M EDTA and extraction with phenol/chloroform and chloroform followed by ethanol precipitation in the presence of 2.5 M ammonium acetate and 4 μg of glycogen carrier. The yield of cDNA was estimated to be approximately 3 μg from starting mRNA template of 15 μg.

Eco RI adapters were ligated onto the 5

ends of the cDNA described above to enable cloning into an expression vector. A 10 μl aliquot of cDNA (approximately 1.5 μg) and 5 μof 65 pmole/μl of Eco RI adapter (Pharmacia LKB Biotechnology Inc.) were mixed with 2 μl 10× ligase buffer (660 mM Tris-HCl pH 7.5, 100 mM MgCl₂), 2 μl of 10 mM ATP and 1 μl of 15.U/μl T4 DNA ligase (Promega Corp., Madison, Wis.). The reaction was incubated 2 hours at 5° C., two hours at 7.5° C., 2 hours at 10° C, and 10 hours at 12.5° C. The reaction was terminated by incubation at 70° C. for 20 minutes.

To facilitate the directional cloning of the cDNA into an expression vector, the cDNA was digested with Xho I, resulting in a cDNA having a 5

Eco RI cohesive end and a 3

Xho cohesive end. The Xho I restriction site at the 3

end of the cDNA had been previously introduced using the ZC6091 primer (SEQ ID NO: 8). Restriction enzyme digestion was carried out in a reaction mixture containing 20 μl of cDNA as described above, 10 μl of 10×H Buffer Xho I (Boehringer Mannheim), 69 μl H₂O, and 1.0 μl of 40 U/μl Xho I (Boehringer Mannheim). Digestion was carried out at 37° C. for 40 minutes. The reaction was terminated by incubation at 70° C. for 10 minutes and chromatography through a 400 pore size gel filtration column (Clontech Laboratories).

The cDNA was ethanol precipitated, washed with 70% ethanol, air dried and resuspended in 14 μl water, 2 μl of ligase buffer (Promega Corp., Madison, Wis.), 2 μl T4 polynucleotide kinase (10 U/μl, Life Technologies). Following incubation at 37° C. for 30 minutes, the cDNA was heated to 65° C. for 5 minutes, cooled on ice, and electrophoresed on a 0.8% low melt agarose gel. The contaminating adapters and cDNA below 0.6 kb in length were excised from the gel. The electrodes were reversed, and the cDNA was electrophoresed until concentrated near the lane origin. The area of the gel containing the concentrated cDNA was excised and placed in a microfuge tube, and the approximate volume of the gel slice was determined. An aliquot of water approximately three times the volume of the gel slice (300 μl) and 35 μl 10×β-agarose I buffer (New England Biolabs) were added to the tube, and the agarose was melted by heating to 65° C. for 15 minutes. Following equilibration of the sample to 45° C., 3 μl of 1 U/μl β-agarose I (New England Biolabs) was added, and the mixture was incubated for 60 minutes at 45° C. to digest the agarose. After incubation, 40 μl of 3 M Na acetate was added to the sample, and the mixture was incubated on ice for 15 minutes. The sample was centrifuged at 14,000×g for 15 minutes at room temperature to remove undigested agarose. The cDNA was ethanol precipitated, washed in 70% ethanol, air-dried and resuspended in 10 μl water.

The resulting cDNA was cloned into the lambda phage vector λZap^(<<) II (Stratagene Cloning Systems) that was predigested with Eco RI and Xho I and dephosphorylated. Ligation of the cDNA to the λZap<< II vector was carried out in a reaction mixture containing 1.0 μl of prepared vector, 1.0 μl of human testis cDNA, 1.0 μl 10× Ligase Buffer (Promega Corp.), 1.0 μl of 10 mM ATP, 5 μl water, and 1.0 μl of T4 DNA Ligase at 15 units/ml (Promega Corp.). The ligation mixture was incubated at 5°-15° C. overnight in a temperature gradient. After incubation, the ligation mixture was packaged into phage using an in vitro packaging extract (Gigapack<< III Gold packaging extract; Stratagene Cloning Systems), and the resulting library was titered according to the manufacturer

s specifications.

The human testis λZAP<< II library was used to infect E. coli host cells (XL1-Blue MRF

strain (Stratagene Cloning Systems), and 1.5×10⁶ plaque forming units (pfu) were plated onto 150-mm NZY plates at a density of about 50,000 pfu/plate. The inoculated plates were incubated overnight at 37° C. Filter plaque lifts were made using nylon membranes (Hybond™-N; Amersham Corp., Arlington Heights, Ill.), according to the procedures provided by the manufacturer. The filters were processed by denaturation in solution containing 1.5 M NaCl and 0.5 M NaOH for 6 minutes at room temperature. The filters were blotted briefly on filter paper to remove excess denaturation solution, followed by neutralization for 6 minutes in 1 M Tris-HCl, pH 7.5, and 1.5 M NaCl. Phage DNA was fixed onto the filters with 1,200 μJoules of UV energy in a UV Crosslinker (Stratalinker<<; Stratagene Cloning Systems). After fixing, the filters were first pre-washed in an aqueous solution containing 0.25× standard sodium citrate (SSC), 0.25% sodium dodecyl sulfate (SDS) and 1 mM EDTA to remove cellular debris and then prehybridized in hybridization solution (5×SSC, 5× Denhardt

s solution, 0.2% SDS and 1 mM EDTA). Heat-denatured, sheared salmon sperm DNA at a final concentration of 100 μg/ml was added. The filters were prehybridized at 65° C. overnight.

A probe was prepared as a PCR product by using oligonucleotide primers designed to amplify the human Zcytor7 coding region. A PCR reaction mixture was prepared containing 2 μl of ZC11526 (SEQ ID NO:9) 2 μl of ZC11,108 (SEQ ID NO:6), 1 μl of an overnight bacterial culture of pSLR7-1, 1 μl of 10 mM dNTP, 10 μl of 10× KlenTaq buffer (Clontech Laboratories), 82 μl water, and 2 μl KlenTaq DNA polymerase (Clontech laboratories). The PCR reaction was run as follows: 94° C. for 1 minute; 30 cycles of 95° C. for 20 seconds, 43° C. for 20 seconds, 68° C. for 1 minute; then held at 68° C. for 10 minutes. The PCR product was re-amplified and gel purified on a 0.8% low melt agarose gel.

Fifty nanograms PCR product was radiolabeled with 32P-α-dCTP by random priming using the MEGAPRIME<< DNA Labeling System (Amersham), according to the manufacturer

s specifications. The prehybridization solution was replaced with fresh hybridization solution containing 1.4×10⁶ cpm/ml labeled probe and allowed to hybridize for 64 hours at 60° C. After hybridization, the hybridization solution was removed and the filters were rinsed in a wash solution containing 0.25×SSC, 0.25% SDS and 1 mM EDTA at 65° C. The filters were placed on autoradiograph film and exposed at −70° C. with intensifying screens for 72 hours.

Examination of the autoradiographs revealed multiple regions that hybridized with labeled probe. Agar plugs were picked from 12 regions for purification. Each agar plug was soaked 2 hours in 0.5 ml of SM solution containing 25 ml 4M NaCl, 10 ml 1M MgSO₄, 25 ml 2M Tris HCl, 5 ml 2% gelatin and 935 ml H₂O and 10% (v/v) chloroform (Sambrook et al. Molecular Cloning: A Laboratory Manual, 2^(nd) ed., Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y., 1989). After incubation, the phages from each plug were diluted 1:1000 in SM. Aliquots of 50 μl were plated on 100 mm plates containing 300 μl of E. coli XL-1 Blue MRF

cells. The plates were incubated overnight at 37° C., and filter lifts were prepared, prehybridized overnight, hybridized overnight with a hybridization solution containing 1.1×10⁶ cpm/ml labeled probe, washed and autoradiographed. Examination of the resulting autoradiographs revealed 10 positive signals. The positive plaques were subjected to an additional round of purification.

The plasmids were excised using an ExASSIST/SOLR<< system (Stratagene Cloning Systems), according to the manufacturer

s specifications. These plasmid inserts were amplified by PCR for size determination. A clone, designated pSLR7-2 was sequenced and determined to have an insert of 3,532 bp in size.

EXAMPLE 2 Northern Blot Analysis

A 970 bp fragment of the Zcytor7 cDNA containing nucleotides 822-1791 was random primer labeled using a MULTIPRIME™ kit (Amersham Corp.). Labeled cDNA was purified from free counts using a push column (Stratagene Cloning Systems). A human RNA master dot blot (Clontech Laboratories) for three hours at 65° C., then hybridized with 10⁶ cpm/ml of labeled cDNA probe. The expression pattern for this blot, which contained RNA samples which had been normalized to the mRNA expression levels of eight different housekeeping genes, was highest in kidney, followed by spinal cord, prostate, and cerebellum.

EXAMPLE 3 Expression of Human Zcytor7 mRNA in Human Tissues

Poly(A)⁺ RNAs isolated from adrenal cortex, adrenal medulla, brain, colon, heart, kidney, liver, lung, ovary, pancreas, prostate, placenta, peripheral blood leukocytes, stomach, spleen, skeletal muscle, small intestine, testis, thymus, thyroid, fetal brain, fetal lung, fetal liver and fetal kidney were hybridized under high stringency conditions with a radiolabeled DNA probe containing nucleotides 822-1791 of (SEQ ID NO: 1). Membranes were purchased from Clontech. The membrane were washed with 0.1×SSC, 0.1% SDS at 50° C. and autoradiographed for 24 hours. The mRNA levels were highest in adrenal cortex, pancreas and prostate with lower levels in testis, stomach, adrenal medulla and thyroid.

EXAMPLE 4 Chromosomal Assignment and Placement of Zcytor-7

Zcytor-7 was mapped to chromosome 6 using the commercially available version of the Whitehead Institute/MIT Center for Genome Research's “GeneBridge 4 Radiation Hybrid Panel” (Research Genetics, Inc., Huntsville, Ala.). The GeneBridge 4 Radiation Hybrid Panel contains PCRable DNAs from each of 93 radiation hybrid clones, plus two control DNAs (the BFL donor and the A23 recipient). A publicly available WWW server (http://www-genome.wi.mit.edu/cgi-bin/contig/rhmapper.pl) allows mapping relative to the Whitehead Institute/MIT Center for Genome Research's radiation hybrid map of the human genome (the “WICGR” radiation hybrid map) which was constructed with the GeneBridge 4 Radiation Hybrid Panel.

For the mapping of zcytor-7 with the, “GeneBridge 4 RH Panel”, 25 μl reactions were set up in a PCRable 96-well microtiter plate (Stratagene, La Jolla, Calif.) and used in a “RoboCycler Gradient 96” thermal cycler (Stratagene). Each of the 95 PCR reactions consisted of 2.5 μl 50× “Advantage KlenTaq Polymerase Mix” (CLONTECH Laboratories, Inc., Palo Alto, Calif.), 2=|1 dNTPs mix (2.5 mM each, PERKIN-ELMER, Foster City, Calif.), 1.25 μl sense primer, ZC 11,131, (SEQ ID NO: 11), 1.25 μl antisense primer, ZC 11,155, (SEQ ID NO: 12), 2.5 μl “RediLoad” (Research Genetics, Inc., Huntsville, Ala.), 0.5=|1 “Advantage KlenTaq Polymerase Mix” (Clontech Laboratories, Inc.), 25 ng of DNA from an individual hybrid clone or control and water is added to bring up the total volume to 25=|1. The reactions were overlaid with an equal amount of mineral oil and sealed. The PCR cycler conditions were as follows: an initial 1 cycle 4 minute denaturation at 94° C., 35 cycles of a 1 minute denaturation at 94° C., 1.5 minute annealing at 63° C. and 1.5 minute extension at 72° C., followed by a final 1 cycle extension of 7 minutes at 72° C. The reactions were separated by electrophoresis on a 3% NuSieve GTG agarose gel (FMC Bioproducts, Rockland, Me.).

The results showed that zcytor-7 maps 795.76 cR from the top of the human chromosome 6 linkage group on the WICGR radiation hybrid map. Relative to the centromere, its nearest proximal marker was CHLC.GATA32BO3 and its nearest distal maker was SGC32063. The use of surrounding markers also helped position zcytor-7 in the 6q22-q23 region on the CHLC chromosome 6 version v8c7 integrated marker map (The Cooperative Human Linkage Center, WWW server: http://www.chlc.org/ChlclntegratedMaps.html) and to 6q22.33-q23.1 on the integrated LDB chromosome 6 map (The Genetic Location Database, University of Southhampton, WWW server:http://cedar.genetics.soton.ac.uk/public_html/).

This is a common breakpoint area in ALL (acute lymphoblastic leukemia) and NHL (non-Hodgkin lymphoma) as well as in AML (acute myelogenous leukemia) and CML (chronic myeloid leukemia). It is interesting to note that the MYB (avian myeloblastosis viral oncogene homolog) gene, which encodes proteins critical for hematopoetic cell proliferation and development, appears to be less than 800 kB from zcytor7. The 6q-deletion breakpoints occur slightly distal to the MYB gene and although the neoplasms show high levels of MYB mRNA, the gene itself appears to be intact. 

1. An isolated polynucleotide encoding a ligand-binding receptor polypeptide, said polypeptide comprising a sequence of amino acids selected from the group consisting of: (a) residues 30 to 250 of SEQ ID NO:2; (b) allelic variants of (a); and (c) sequences that are at least 80% identical to (a) or (b).
 2. An isolated polynucleotide according to claim 1 wherein said polypeptide further comprises a transmembrane domain.
 3. An isolated polynucleotide according to claim 2 wherein said transmembrane domain comprises residues 251 to 274 of SEQ ID NO:2, or an allelic variant thereof.
 4. An isolated polynucleotide according to claim 2 wherein said polypeptide further comprises an intracellular domain.
 5. An isolated polynucleotide according to claim 4 wherein said intracellular domain comprises residues 275 to 553 of SEQ ID NO:2, or an allelic variant thereof.
 6. An isolated polynucleotide according to claim 1 which is a DNA as shown in SEQ ID NO:1 from nucleotide 339 to nucleotide
 1009. 7. An isolated polynucleotide according to claim 1 wherein said polypeptide further comprises an affinity tag.
 8. An isolated polynucleotide according to claim 7 wherein said affinity tag is polyhistidine, protein A, glutathione S transferase, substance P, or an immunoglobulin heavy chain constant region.
 9. An isolated polynucleotide according to claim 1 wherein said polynucleotide is DNA.
 10. An isolated polynucleotide encoding a polypeptide selected from a group defined SEQ ID NO:2 consisting of residues 1 to 250, residues 1 to 274, residues 1 to 553, residues 2 to 250, residues 2 to 274, residues 2 to 553, residues 251 to 274, residues 251 to 553 and residues 275 to
 553. 11. An expression vector comprising the following operably linked elements: a transcription promoter; a DNA segment encoding a ligand-binding receptor polypeptide, said polypeptide comprising a sequence of amino acids selected from the group consisting of: (a) residues 30 to 250 of SEQ ID NO:2; (b) allelic variants of (a); and (c) sequences that are at least 80% identical to (a) or (b); and a transcription terminator.
 12. An expression vector according to claim 11 wherein said polypeptide further comprises a signal sequence.
 13. An expression vector according to claim 11 wherein said polypeptide further comprises a transmembrane domain.
 14. An expression vector according to claim 11 wherein said transmembrane domain comprises residues 251 to 274 of SEQ ID NO:2, or an allelic variant thereof.
 15. An expression vector according to claim 13 wherein said polypeptide further comprises an intracellular domain.
 16. An expression vector according to claim 15 wherein said intracellular domain comprises residues 275 to 553 of SEQ ID NO:2, or an allelic variant thereof.
 17. An expression vector comprising the following operably linked elements: (a) a transcription promoter; (b) a DNA segment encoding a chimeric polypeptide, wherein said chimeric polypeptide is comprised of a first portion and a second portion joined by a peptide bond, said first portion consisting essentially of a ligand binding domain of a receptor polypeptide selected from the group consisting of: (i) a receptor polypeptide as shown in SEQ ID NO:2; (ii) allelic variants of SEQ ID NO:2; and (iii) receptor polypeptides that are at least 80% identical to (i) or (ii), and said second portion consisting essentially of an affinity tag; and (c) a transcription terminator.
 18. An expression vector according to claim 17 wherein said affinity tag is an immunoglobulin F_(c) polypeptide.
 19. A transformed or transfected cell into which has been introduced an expression vector according to claim 11, wherein said cell expresses a receptor polypeptide encoded by the DNA segment.
 20. The cell of claim 19 wherein said cell is prokaryotic.
 21. The cell of claim 19 wherein said cell is eukaryotic. 